There are two basic techniques used in digital-to-analog convertors (DACs). These are the sigma-delta (.SIGMA.-.DELTA.) and resistive or capacitive divider techniques.
For the resistive divider technique, a relatively simple DAC comprises a digital buffer register and a resistor network for simultaneously coupling outputs of different resister stages through a common circuit node to an output connection. For these types of devices, the output accuracy is greatly influenced by the ratios among the resistances of the weighted resistors of various sizes employed in the resistor networks. High levels of accuracy are difficult to achieve and expensive, particularly with faster 16 bit processing.
The .SIGMA.-.DELTA. technique is attractive because it achieves high resolution by precise timing instead of precisely matched on-chip components (resistors). In addition, the expertise needed to produce thin film, laser trimmed analog components are difficult to obtain; whereas, high speed digital switching capability is commonplace in the semiconductor industry.
A basic .SIGMA.-.DELTA. DAC receives a digital signal which is summed with the inverse feedback of the analog output signal (after being reconverted to a digital signal) to provide an error signal. The error signal is then processed through an integrator and a comparator to provide the analog output signal. The analog signal is also processed through an analog-to-digital convertor (ADC) to provide the feedback signal.